What a plugged transmission vent can cause?

A clogged vent plug will allow pressure to build up inside the transmission from the heat of friction or ambient temperature even if it's just sitting. That increased pressure forces outward on every surface in the transmission. With no pressure release, it can force the transmission oil or fluid out the axle shaft seals until it is too low for safe operation. Early transmission failure can result, as well as unsightly puddles underneath the car.